The Cell Signaling in Vertebrate Development Section is taking genetic and molecular approaches to understand the cellular and molecular targets of Wnt signaling molecules during early embryogenesis and tumorigenesis. The laboratory is focused on a complex morphogenetic process known as gastrulation, during which the mesoderm and endoderm germ layers are formed and the body plan is established. Gastrulation begins with the formation of the primitive streak (PS), a transient developmental structure that forms at the posterior terminus and converts pluripotent epiblast stem cells into mesoderm and endoderm progenitors. We have shown that seven of the nineteen mammalian Wnt genes are expressed in the PS, along with members of several other growth factor families. These results indicate that the PS is a signaling center and suggest that Wnts may regulate stem cell self-renewal or differentiation. Several pathways are known to transduce Wnt signals, including the Wnt/?catenin and Wnt/Planar Cell Polarity (PCP) pathways. This project specifically addresses the role that the Wnt/?catenin pathway plays in gastrulation. We have previously generated a transgenic reporter of Wnt/?catenin signals in vivo (the BATlacZ mouse) and shown that the pathway is highly active in the PS, primarily functioning to transduce Wnt3a signals (Nakaya et al., 2005). Through the use of the T-Cre driver, which expresses Cre recombinase specifically in the PS (obtained in collaboration with my colleague Mark Lewandoski), and conditional loss and gain of function alleles of ?catenin, we have shown genetically that Ctnnb1 (which encodes ?catenin) transduces Wnt3a signals in the PS, is necessary for mesoderm formation and segmentation, and regulates segmentation by activating a network of interacting target genes that promote mesodermal fates and position boundary determination genes in the anterior presomitic mesoderm (PSM) (Dunty et al., 2007a, Development, in press). These results suggest that Wnt3a and ?catenin may be necessary for the specification of an embryonic mesodermal stem cell. The conditional Ctnnb1 alleles have also proven to be excellent genetic tools for the characterization of novel target genes of Wnt3a. A major goal of the laboratory is to elucidate the target genes and transcriptional networks activated by Wnt3a and ?catenin. We have generated genome-wide transcriptional profiles of wildtype (wt) and Wnt3a-/- embryos and identified 150 differentially expressed genes including several previously characterized direct Wnt/?catenin target genes (Dunty et al., 2007b, in prep). Remarkably, over a third of these genes encode transcriptional regulators. We have performed in situ hybridization screens to examine the expression of these putative target genes in embryos and in gastrointestinal tumors and have identified several novel target genes that are expressed in the embryo, or in both the embryo and adult gastrointestinal tumors. These studies led to the identification of a novel transcriptional corepressor, Ripply2, which is expressed during somitogenesis (Biris et al., 2007, Dev. Dyn., in press), and is transcriptionally regulated by Wnt3a and ?catenin suggesting a likely mechanism for the control of segmentation by Wnts (Dunty et al., 2007a, Development, in press). We have also discovered that a previously identified gene encoding the bHLH transcription factor Mesogenin1, known to be important for embryonic development, is a direct target gene of Wnt3a and ?catenin during mesoderm formation. Interestingly, we show through double mutant analyses that Mesogenin is a feedback suppressor of Wnt3a, downregulating Wnt3a to promote the maturation of mesodermal stem cells to a committed presomitic mesoderm fate. Finally, we have identified the Zn finger transcription factor Sp5 as a downstream target gene of Wnt3a. Although other groups have previously shown that it is an embryonic Wnt target, we have shown that Sp5 is highly expressed in adult gastrointestinal tumors. Our preliminary functional analyses suggest that Sp5 is an important effector of Wnt signaling during tumorigenesis. The loss of Sp5 in animal models of familial adenomatous polyposis (FAP) caused by mutation in tumor suppressors in the Wnt/bcatenin pathway leads to the formation of fewer GI tumors. Detailed studies of these transcription factors and their regulatory relationships has begun to elucidate the key governing principles of the transcriptional regulatory cascades controlling mammalian Wnt signaling during development and disease and have led to the identification of new potential targets for therapy